Particle counters generally have a carpet of light, through which a mostly individualised particle stream is passed, each particle generating stray light as it passes, which is detected by a light sensor. In order to improve measurability, the particles usually first pass through a condensation unit, in which condensation droplets grow on particles. Due to the greater size and the uniformity of the condensation droplets, counting is enabled as opposed to a direct measurement of the particles.
Due to the Gaussian distribution of light intensity of the carpet of light, a passing particle generates an equally Gaussian signal pulse. In an optimal case, each individual particle provides a single stray light pulse, which corresponds to a temporal profile of sporadically occurring signal pulses. The statistic probability distribution of discrete events may here be determined via a Poisson distribution.
If the temporal distance between particles is too small, coincidence will occur, wherein the signal pulses generated by the particles following one another in quick succession superimpose to form a single signal pulse and can no longer be separated as two individual events.
The light sensor generates an analogue measurement signal that is evaluated for detecting and counting the signal pulses. Usually, a threshold value is defined here, wherein the generated signal pulse of a passing particle is detected by a comparator member and the particle is regarded as having been detected when this threshold value is exceeded. In a case where a threshold value is selected to be too high it may occur that signal pulses that are too small are falsely not counted. However, if the threshold value is selected to be too low, then signal pulses that are too close together and partially overlap (coincidences) are detected only as one single signal pulse, and therefore some signal pulses are not detected. Also background noise may lead to false results in the case of too low a threshold value.
What is particularly disadvantageous is if the measured pulse ensemble has such a drift behaviour that said ensemble, during the measurement, moves out of the measurement range defined by the threshold value. Signal evaluation via threshold values may here unnoticeably lead to a wrong result.
Usually, signal evaluation is carried out in the analogue domain because the signal pulses in the measurement signal are very short, usually in a range of 80 to 200 ns, so that digitalisation requires a very high sampling rate. Although AD converters are available that allow a sufficiently high sampling rate even for very small signal pulses, evaluation by means of threshold values in the digital domain has however no substantial advantages over the analogue domain, so that in general evaluation in the analogue domain provides for a simpler and therefore more preferred solution.
Thus, there is a need in the prior art for devices and methods for detecting signal pulses, which allow an improved detection of coincidences and an enhanced counting accuracy and which provide a possibility of detecting, evaluating and compensating for a drift behaviour of the measurement signal.